1. Field of Invention
The present invention relates to a display device having an electro-optical device such as a liquid crystal device, or the like, in which transmittance (reflectance) varies with applied voltage.
2. Description of Related Art
Electronic equipment incorporating such a display device includes, for example, a projection display device in which three liquid crystal devices (LCD) for performing optical modulation of each chromatic light of red (R), green (G), and blue (B) are respectively used as a light valve (L/V). FIG. 16 shows applied voltage-transmittance (V-T) characteristics inherent in each of the liquid crystal devices for performing optical modulation of each chromatic light of R, G, and B (in which twisted nematic liquid crystal is used). In FIG. 16, although the range for applying voltage to the liquid crystal can be set between 0V and 6V, both the white level region in which transmittance is approximately 100% and the black level region in which transmittance is approximately 0% are saturated. Accordingly, such an arrangement is provided so that amplitude of the voltage for application to the liquid crystal is limited to a level of approximately 3.8V so as to prevent the white level region and the black level region from becoming saturated. That is, in this arrangement, direct current bias (DC bias) of a picture signal is adjusted in such a manner that voltage which does not produce saturation in the regions of the white level and black level is applied to each liquid crystal. This is referred to as brightness adjustment.
In FIG. 17, the V-T characteristics of the chromatic lights R, G, and B present a mutually different inclination, and there is shown variations in the voltage which produces saturation in the regions of the white level and the black level among the colors. In order to reduce the variations, a gain of a picture signal is adjusted, which is referred to as gain adjustment. Furthermore, in the liquid crystal device (LCD), transmittance is determined by the ratio of light amounts which are transmitted through a liquid crystal panel having liquid crystal between a pair of substrates and through a polarizer disposed on at least one side of the liquid crystal panel. Reflectance is substituted for transmittance in a reflection-type electro-optic device.
FIG. 17 shows a relationship of a gray-scale value and transmittance of a digital input signal after brightness adjustment and gain adjustment. As shown in FIG. 17, in the black level region, there is a slight change in transmittance with respect to changes of gray-scale values, so that satisfactory resolution cannot be obtained.
In order to obtain ideal gamma characteristics (ideal xcex3 characteristics) as shown in FIG. 17, gamma correction characteristics shown in FIG. 18 are used for correcting a digital picture signal. When the liquid crystal device is driven based on the signal subjected to gamma correction performed according to the characteristics shown in FIG. 18, characteristics close to the ideal gamma characteristics in FIG. 17 can be obtained as shown in FIG. 19. Thus, conventionally, in order to obtain gamma correction characteristics, it is essential to perform brightness adjustment and gain adjustment in advance.
Consequently, a display device incorporating a conventional liquid crystal device requires a circuit arrangement as shown in FIG. 15. In FIG. 15, a picture signal is converted into a digital picture signal by an analog-to-digital (A/D) converter 10 to perform signal-processing including gamma correction and digital-to-analog (D/A) conversion by a picture signal processing circuit 20. The picture signal processing circuit 20 includes ASIC 22 having a gamma correction circuit and a D/A converter 24. An amplifier 30 performs gain adjustment of the analog picture signal, and then a bias adjustment circuit 40 performs DC bias adjustment (brightness adjustment) of the signal so as to send it to a liquid crystal device 50.
A CPU 60 shown in FIG. 15 serves as a controller of the display device. The CPU 60 controls gamma correction as follows. First, the V-T characteristics which are inherent in the liquid crystal device 50 shown in FIG. 16 are actually measured. Next, the CPU 60 controls gain adjustment performed by the amplifier 30 through a gain controller 80, and also controls DC bias performed by the bias adjustment circuit 40 through a brightness controller 90 to obtain the V-T characteristics shown in FIG. 17. An EEPROM 70 stores the obtained the V-T characteristics, the gain adjustment data, and the brightness adjustment data, from which the V-T characteristics are obtained. Then, the CPU 60 calculates gamma correction characteristics shown in FIG. 18, based on the V-T characteristics stored in the EEPROM 70, which is shown in FIG. 17, and predetermined ideal gamma characteristics, and sets the gamma correction characteristics, for example, as table information, in a gamma correction circuit inside the ASIC 22. Accordingly, in order to obtain the V-T characteristics as shown in FIG. 19 in the liquid crystal device 50, such an arrangement is provided so that a picture signal input from the A/D converter 10 is corrected according to table information so as to perform gain adjustment and brightness adjustment by the amplifier 30 and the bias adjustment circuit 40 according to the gain adjustment data and the brightness adjustment data stored in the EEPROM 70.
Therefore, it is essential for the conventional display device described above to perform such gain adjustment and brightness adjustment in advance before determining gamma correction characteristics, as shown in FIG. 18, in the gamma correction circuit inside the ASIC 22. Such gain adjustment and brightness adjustments are extremely complicated since they vary among liquid crystal devices. In addition, precise adjustment is necessary, since misadjustment directly affects image quality. More specifically, since a projection display device which synthesizes chromatic lights modulated by a plurality of liquid crystal devices for projection requires mutual adjustment between the liquid crystal devices, performing gamma correction requires very complicated work.
In addition, since the conventional display device performs brightness adjustment, the transmittance range of the liquid crystal device is narrowed, leading to reduction in contrast and darkening of the display screen. In other words, the conventional brightness adjustment and gain adjustment result in the transmittance, for example, of 3% for the black level and of 97% for the white level in the liquid crystal device. This allows contrast reduction and increased darkening of the screen compared with a device which uses the whole transmittance range of 0 to 100%, as described above.
Accordingly, it is an object of the present invention to provide a display device and electronic equipment which can yield the brightness and contrast of an electro-optical device as close to the best characteristics that the device can offer.
It is another object of the present invention to provide a display device and electronic equipment in which gain adjustment and brightness adjustment are not necessary.
It is another object of the present invention to provide a display device and electronic equipment which can reproduce ideal input-output characteristics in terms of gamma correction and color temperature.
According to the present invention, there is provided a display device including an electro-optical device in which transmittance changes according to voltage applied to an electro-optical material, a digital gamma correction circuit for performing gamma correction of a digital picture signal, a digital-to-analog conversion circuit for converting the digital picture signal corrected by the digital gamma correction circuit into an analog picture signal, and an amplifier for amplifying the analog picture signal, in which voltage is applied to an electro-optical material based on the output of the amplifier, and in which the digital gamma correction circuit converts the digital picture signal of n bits into a digital picture signal of N bits (Nxe2x89xa7n+2) based on gamma correction characteristics predetermined by applied voltage-transmittance characteristics inherent in the electro-optical device.
In addition, according to the present invention, there is provided a gamma correction method for correcting applied voltage-transmittance characteristics inherent in an electro-optical device in which light transmittance changes depending on voltage being applied to the electro-optical material, in which gamma correction is performed on a digital picture signal, the gamma-corrected digital picture signal is converted into an analog picture signal that is amplified, voltage is applied to the electro-optical material based on the amplified analog picture signal, and when the gamma correction is performed, the digital picture signal of n bits is converted into a digital picture signal of N bits (Nxe2x89xa7n+2) based on a gamma correction characteristics predetermined according to applied voltage-transmittance characteristics inherent in the electro-optical device.
In applied voltage-transmittance characteristics inherent in the electro-optical device, in the case of a normally black mode, there is a saturated situation (see FIG. 3) in which changes in transmittance are small with respect to changes of applied voltage on the black level side of transmittance 0%. Consequently, gamma correction characteristics for compensating for this show a sharp curve on the black level side and a great deal of gray-scale data is used for correcting in the region (see FIG. 4). In the case of a normally white mode, in contrast, since the white level side of transmittance 100% becomes saturated, a great deal of gray-scale data is used for correction in the region. In other words, in either display modes, V-T characteristics change less in the proximity of transmittance 0% or 100%. As a result, in order to change a display gray scale at an equal level by performing gamma correction in the region, voltage change for a picture signal in the region is set to be small so as to allow transmittance to be more linearly changed. It is necessary for making the voltage change for the picture signal small to use more gray-scale data of the digital picture signal in the region.
As a result, the gray-scale data amount assigned to the region including halftones of luminance 10% to 90% is reduced. In fact, when the number of bits for inputting to the digital gamma correction circuit is set as n bits, for example 8 bits, it is found that even though the number of bits for outputting is set to n or n+1, sufficient gray-scale data cannot be assigned to the region including halftones originally having data of about 200 levels of gray scale with 10% to 90% of luminance (see the case of 9-bit outputting in FIG. 4). The present invention solves the above problem by making the number of outputting bits n+2 bits or more (see the case of 10-bit outputting in FIG. 4), in the digital gamma correction circuit.
As described above, the present invention permits ensuring of the number of bits for a digital picture signal assigned to the region of halftones, even in the case of using a region with less change ratio in applied voltage-transmittance characteristics inherent in the electro-optical device. Thus, the present invention permits the range for using applied voltage-transmittance characteristics inherent in the electro-optical device to be enlarged. This can lead to provision of an image with satisfactory brightness and high contrast.
Accordingly, the present invention allows gamma correction characteristics to be determined by applied voltage-transmittance characteristics inherent in the electro-optical device in the entire transmittance range of 0% to 100%. Consequently, this requires neither the conventional brightness adjustment nor gain adjustment.
In order to make the conventional brightness adjustment and gain adjustment unnecessary, the digital gamma correction circuit performs at least one (preferably both) of bias adjustment and gain adjustment of a picture signal to perform conversion of a digital picture signal. This allows the amplifier to have no variable resistor being bias adjustment means and gain adjustment means.
Moreover, in the present invention, the amplifier outputs a picture signal to reverse the polarity of voltage applied to the electro-optical material in a specified cycle. The digital gamma correction circuit outputs the digital picture signal, and a digital polarity inverting circuit digitally reverses the polarity of the digital picture signal in the specified cycle. In addition, the digital-to-analog converter outputs the analog picture signal, and an analog polarity inverting circuit reverses the polarity of the analog picture signal in an analog form in the specified cycle.
The present invention permits voltage of a first polarity and a second polarity to be applied to an electro-optical material in a specified cycle. This allows arrangement of the digital polarity inverting circuit for digitally inverting the polarity of the digital picture signal output from the digital gamma correction circuit in the specified cycle. In addition, this also allows arrangement of the analog polarity inverting circuit for inverting the polarity of the analog picture signal output from the digital-analog converter in an analog form in each specified cycle.
When such a polarity inversion is performed, it is preferable that voltage output from the amplifier in achieving either one of the maximum transmittance and the minimum transmittance by the electro-optical device be substantially equal in both voltage applications using a first polarity and a second polarity. The equal voltage is a central potential of an amplitude of voltage from the amplifier. In this state, when the range for using applied voltage-transmittance characteristics inherent in an electro-optical device is increased, as described above, the amplitude of output from the amplifier is also increased. However, using the central potential of the amplitude of output from the amplifier, for example, as a potential for the white level mode in both cases of a first polarity and a second polarity permits voltage amplitude to be minimal.
Furthermore, the electronic equipment of the present invention has a plurality of electro-optical devices in which light transmittance changes based on voltage applied to an electro-optical material, and lights modulated by a plurality of the electro-optical devices are synthesized for display, in which each of the electro-optical devices has a digital gamma correction circuit for executing gamma correction of a digital picture signal, a digital-to-analog conversion circuit for converting the digital picture signal corrected by the digital gamma correction circuit into an analog picture signal, and an amplifier for amplifying the analog picture signal, in which voltage is applied to the electro-optical material based on output from the amplifier, and in which the digital gamma correction circuit converts the digital picture signal of n bits into a digital picture signal of N bits (Nxe2x89xa7n+2) based on gamma correction characteristics predetermined by applied voltage-transmittance characteristics inherent in the electro-optical device.
The present invention allows the range for using applied voltage-transmittance characteristics inherent in the electro-optical device to be increased. This allows each of the formed images to be brighter and be a high contrast image by a plurality of the electro-optical device. Therefore, a displayed image of a projection display device becomes brighter and more high contrast, since this type synthesizes chromatic lights of R (red), G (green), and B (blue) modulated by a plurality of the electro-optical devices and forms an image made of the synthesized light on the screen for display.
When the chromatic lights modulated by the respective electro-optical device are synthesized, in the case where input gray-scale data are mutually equal, it is necessary for the transmittance of V-T characteristics of the electro-optical device to be in a certain ratio relationship (in which the transmittance with respect to gray-scale data needs to be equal, or even if gray-scale levels change, transmittance needs to be equal), otherwise chromatic balance of the synthesized image is lost in response to gray-scale changes. In the present invention, however, the digital gamma correction circuit converts into a digital picture signal in such a manner that the gray-scale data and the curve of transmittance characteristics in the electro-optical device corresponding to the data are substantially equal or similar between a plurality of the electro-optical devices, thereby, achieving a stabilization of chromatic balance regardless of gray scale levels.
The digital gamma correction circuit performs at least one of bias adjustment and gain adjustment of a picture signal applied to the electro-optical material to convert the picture signal into a digital picture signal. This permits the amplifier to have no variable resistor being bias adjustment means and gain adjustment means.
A plurality of the electro-optical devices respectively modulate mutually different chromatic lights and the digital gamma correction circuit which corresponds to each of the plurality of electro-optical devices performs correction of color temperature which is presented by a synthesized light made of chromatic lights modulated by the respective electro-optical devices. More particularly, the digital gamma correction circuit which corresponds to each of the plurality of electro-optical devices allows a plurality of the electro-optical devices to mutually adjust slopes of the transmittance characteristic curves of the electro-optical devices with respect to the gray-scale data so as to perform color temperature correction. In other words, mutual adjustment of the inclination of the V-T characteristic curve of each electro-optical device between the chromatic lights permits differentiation in the color temperatures of synthesized light, so that the digital gamma correction circuit of the present invention can achieve correction that includes the color temperature correction of displayed colors.
In the present invention, a picture signal is equivalently regarded as an image signal, and transmittance ratio is optical reflectance in a reflection type electro-optical device.